ARM: enable vmalloc saving

For some targets that have less vmalloc space this can be increased by
enabling config ENABLE_VMALLOC_SAVING. With this config we can reclaim
virtual mappings which remains unused because of non hlos carveout
reservations in lowmem.

Select the default method of reclaiming virtual memory as vmalloc
saving.

Change-Id: I249992871babe8c64c34d52ef43bbc7c81636d47
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>